Fluorescein, a fluorescent substance known from the 19th century, can be excited at around 500 nm in an aqueous solution, and has a high quantum yield. For this reason, the substance has been commonly used as a fundamental scaffold of fluorescent probes. For example, fluorescein is used as a fundamental nucleus of a fluorescent probe for nitrogen monoxide (Japanese Patent Laid-Open No. 10-226688(1998)), a fluorescent probe for zinc (WO 01/62755), or the like.
6-Hydroxy-9-phenylfluorone, in which the carboxyl group of fluorescein is substituted with hydrogen atom, has a lower fluorescence quantum yield. Accordingly, the carboxyl group is believed to have a role in characteristics as a fluorophore of fluorescein (Lindqvist, L., et al., J. Chem. Phys., 44, 1711-12, 1966). From the above reason, the carboxyl group is preserved in fluorescein derivatives proposed so far to avoid deterioration of the fluorescent property of fluorescein. Therefore, almost no compound is known wherein the carboxyl group is converted to other functional group.